When someone goes shopping at Amazon.com, is the website indeed recommending to them the product with the best possible price?

That’s the question ProPublica explored in a probe of products available from the e-commerce giant that suggested Amazon uses its computer algorithms to push many products that it sells itself over those of other retailers, even when those items offered by outside sellers on its site cost less than those from Amazon.

In its report, ProPublica said it spent several weeks tracking 250 products that were frequently purchased via Amazon to determine what items most often showed up in the so-called “buy box” that Amazon suggests as the first thing to purchase when a person does a product search. ProPublica said that of those products, Amazon put its own products, and those of other companies paying for Amazon’s services, in a more-favorable purchase position about three-quarters of the time, even when those items cost more than those offered by third-party retailers.

The biggest factor was usually shipping costs, which Amazon did not always prominently disclose in relation to its own items.

ProPublica highlighted a search it had done for Loctite super glue, in which it found two outside retailers offering the product on Amazon for between $6.75 and $7.27 with free shipping. However, those options were supplanted as the top selection by the same Loctite glue sold directly by Amazon for $7.80, with shipping costs of $6.51, costs that weren’t revealed until clicking onto the next link to bring up another page in the shopping process.

An Amazon spokesperson said, “With Prime and Super Saver Shipping, which requires no membership and ships orders above $49 for free, the vast majority of our items ordered – 9 out of 10 – can ship for free. The sorting algorithms the article refers to are designed for that 90 percent of items ordered, where shipping costs do not apply.”

Amazon Prime members also receive free subscriptions to the company’s Prime video-streaming service.

Analyst Michael Pachter, of Wedbush Securities, was critical of the ProPublica report, calling its determinations “lame.”

“I think it’s ludicrous to assume that anyone tech-savvy enough to shop on the internet is too stupid to know how to sort by price,” Pachter said. “The conclusion that ‘most Amazon shoppers end up clicking ‘add to cart’ for the offer highlighted in the buy box’ is unsupported, and I find it unsupportable.”

Pachter said that while Amazon does favor its own listed products, which may end up costing more than those of other retailers when search listings are given by product relevance, “There is no favoritism when the consumer sorts by ‘Price: Low to High’. Consumers should be smart enough to sort by price, particularly those who are smart enough to read ProPublica’s posts.”